ghost shifting

[emily_in_nc]

I am getting ghost shifting or skipping (I don't think it's actually changing gears, it just sounds like it) when I'm going downhill in my small cogs in back (middle crank in front) on my mountain bike. Not under load. Shifting is fine. The cassette is only a year old and a good one (XTR). I don't ride a lot of miles in a given year, so I am sure I haven't worn it out yet. Does this sound like a chain problem? Bent rear derailleur?

One more thing -- I notice it at the beginning of a ride, but then it seems to stop. Odd.

Any ideas appreciated -- thanks much!

[aicabsolut]

Are you in those gear combos at any other time? If so, do you get the same problem happening? It could be more of a cross chain / chainline issue than something that would manifest under load. But it may get worse under load, if you use those gears on flatter ground. I don't know why it'd go away, though. Hmm. Do you get any front derailleur rub?

[wackyjacky1]

Thanks for the info & advice. I was too tired to stop by the bike shop after work on Fri. and on Sat., so I'll ride over there on Monday and see what they can do.

Mucho appreciado!

[emily_in_nc]

Are you in those gear combos at any other time? If so, do you get the same problem happening? It could be more of a cross chain / chainline issue than something that would manifest under load. But it may get worse under load, if you use those gears on flatter ground. I don't know why it'd go away, though. Hmm. Do you get any front derailleur rub?

No, no front derailleur rub at all. I normally use only the middle front ring on my MTB unless the terrain gets really steep, and I was not aware that any gear in back was considered cross-chaining when used with the middle crank of a triple. Perhaps I should be shifting to the larger ring in back when using the smallest cogs in back? I only use those smaller cogs for descending and on the flats. The riding I do is very rolling, so it's much easier to stay in the middle cog all the way than try to go back and forth in the front for short distances, but if that is what is causing the problem, I can definitely give it a try.

Thanks!

[aicabsolut]

It could be. Looking at how to test the indexing for triples, you tighten the cable such that you don't get rubbing in the middle ring / large cog combo. I've seen a bit of rubbing going on in the middle ring / small cog combo a lot (it's my friend's favorite gear).

The chain is pretty slack at this point, though not as much as in small ring/small cog. I wonder if you're getting symptoms of a stiff link. What happens if you put it in the small ring/small cog (you may have to use some FD trim to keep the chain from grinding around on the FD). You could also just try trimming when in the middle ring / small cogs in case you do get some slight rubbing when the frame flexes.

I'm pretty perplexed, though.

If it's trying to autoshift in the small cogs, can you tell which direction? Does it try to jump up or down? You might want to recheck your upper limit.

The first place I'd look would be to see if there's a stiff or bent link or if your derailleur hangar is bent.

[emily_in_nc]

It could be. Looking at how to test the indexing for triples, you tighten the cable such that you don't get rubbing in the middle ring / large cog combo. I've seen a bit of rubbing going on in the middle ring / small cog combo a lot (it's my friend's favorite gear).

The chain is pretty slack at this point, though not as much as in small ring/small cog. I wonder if you're getting symptoms of a stiff link. What happens if you put it in the small ring/small cog (you may have to use some FD trim to keep the chain from grinding around on the FD). You could also just try trimming when in the middle ring / small cogs in case you do get some slight rubbing when the frame flexes.

I'm pretty perplexed, though.

If it's trying to autoshift in the small cogs, can you tell which direction? Does it try to jump up or down? You might want to recheck your upper limit.

The first place I'd look would be to see if there's a stiff or bent link or if your derailleur hangar is bent.

Thanks for all the great ideas, aicabsolut -- I'll check it out (to the best of my ability) this weekend when I have a bit more time.

I did ride tonight and noticed the skipping sound while in the two smallest cogs in back and middle in front, pedaling on the flats. I shifted to the large ring in front, and the problem stopped. That may mean it's not a bad chain link, I'd think.

From then on, I always shifted to the large ring when I needed to use higher gears, and things were fine.

I'd still like to get to the root of the problem, though, in case something needs adjusted/changed.

Thank you!

[aicabsolut]

Well a stiff link would be most noticeable in the smallest ring (according to Sheldon Brown). Honestly, I have no experience with bad links other than when my BF broke a link on a brand new chain, but then I could see one plate sticking out, making a click sound each time it went through the wheels in the derailleur.

That it shifts and rides fine in the small cogs while you're in the big ring does tell me that your H limit screw is set right and that your cassette is probably ok.

I'm thinking that maybe your chain is probably getting too old for you to be cross chaining in the middle ring, even if it is harder to really cross chain on a triple. Next, instead of shifting up to the big ring, try using a little FD trim when you want to use the small cogs/middle ring combo (like a 1/2 shift with the front shifter). See what happens.

[emily_in_nc]

I'm thinking that maybe your chain is probably getting too old for you to be cross chaining in the middle ring, even if it is harder to really cross chain on a triple. Next, instead of shifting up to the big ring, try using a little FD trim when you want to use the small cogs/middle ring combo (like a 1/2 shift with the front shifter). See what happens.

I'm going to measure my chain for stretch too.

I am not sure I can do a half-shift in the front. These are mountain-bike shifters (XTR rapid-fire). I could always half-shift on my road bike with either Shimano or Campy shifters but haven't noticed that I can do that on my mountain bike.

Thanks!